1. Field of the Invention
The present invention relates to an improved hydrocracking process.
2. Description of the Prior Art
Hydrocracking is a well known process for upgrading hydrocarbon fractions. See, for example, Hydrocarbon Processing, Sept. 1976, pages 121-128. In the hydrocracking process, a hydrocarbonaceous oil is contacted in the presence of added hydrogen and a hydrocracking catalyst to produce lower boiling hydrocarbons. When the hydrocarbonaceous oil feed for the hydrocracking process comprises a high nitrogen content, the feed is usually hydrorefined in the presence of added hydrogen and a hydrorefining catalyst to decrease the nitrogen content of a feed prior to the hydrocracking stage.
U.S. Pat. No. 3,055,823 discloses a process which comprises hydrorefining a petroleum distillate boiling in the range of 200.degree. to 850.degree. F., preferably, in the range of 325.degree. to 650.degree. F. to denitrify the distillate, removing ammonia from the hydrorefined effluent, heat treating the remaining hydrorefined effluent, and hydrocracking the heat treated effluent. In the heat treating zone, the hydrorefined effluent is passed through a siliceous cracking catalyst at conditions below which significant cracking occurs, i.e. below 650.degree. F.
U.S. Pat. No. 3,730,875 discloses conversion of asphaltene-containing heavy hydrocarbonaceous oils, i.e., more than 10 volume percent boiling above 1050.degree. F., in a combination process comprising catalytic hydrogenation, non-catalytic hydrogenative thermal cracking and catalytic hydrocracking.
It has now been found that subjecting the hydrocracked hydrocarbonaceous bottoms fraction in the absence of hydrogen to a heat treatment and recycling the heat treated bottoms to the hydrocracking zone will increase the selectivity of the hydrocracked products to components boiling in the range of about 300.degree. to about 700.degree. F.
All boiling points to which reference is made herein are atmospheric pressure boiling points unless otherwise specified.